Van Helsing and his intrepid band of vampire hunters might have disposed of Bram Stoker's creation Dracula more than a century ago, but a sequel to the novel by Stoker's great grand-nephew will see them under attack from the undead once again.

Dacre Stoker delved into his ancestor's handwritten notes on the original Dracula novel to pen his sequel, Dracula: The Un-Dead - the original name for Dracula before an editor changed the title. The novel, out next October, draws on excised characters, existing character back-stories and plot threads that were cut from Stoker's original novel, first published 111 years ago.

The new book is set in London in 1912, a quarter of a century after the Count apparently "crumbled into dust". Vampire-hunter Van Helsing's protégé Dr Seward is now a disgraced morphine addict, and Quincey, the son of Stoker's hero Jonathan, has become involved in a troubled theatre production of Dracula, directed and produced by Bram Stoker himself. The play plunges Quincey into the world of his parents' terrible secrets, but before he can confront them his father is found murdered, impaled in Piccadilly Circus.

The original is written in classic epistolatory form, alternating between different narrators; the sequel adopts a more direct storytelling route. "[This] makes it more immediately accessible to a modern thriller readership, while remaining faithful to the spirit and atmosphere of the Victorian original," said publisher Jane Johnson of HarperCollins UK.

The book has caused a storm in the publishing world, selling for more $1m to Dutton US, HarperCollins UK and Penguin Canada. A film version is also in the works, with shooting expected to begin next June.

Dacre Stoker, who formerly coached the Canadian Olympic Pentathlon team and now lives in the US, is writing the novel with Dracula historian Ian Holt, a screenwriter and member of The Transylvanian Society of Dracula. The Un-Dead is the first Dracula story to be fully authorised by the Stoker family since the 1931 film starring Bela Lugosi.

Stoker, speaking to guardian.co.uk from a fishing trip in Tennessee, said he had initially been "a little sceptical" about the project to resurrect Bram Stoker's original themes and characters, which was dreamed up by Holt. "Growing up, all the Stokers in my generation were pretty blasé about the fact we were related to this great horror writer. At Halloween we'd get all these comments about 'are we going to get bitten if we go round to the Stokers?'," he said, admitting that he only got around to reading his great grand-uncle's novel when he went to college. "But Ian seemed to be the real deal."

Stoker and Holt say they have each written equal amounts of the novel. "When we started I was worried because Dacre had never written a novel before, but he was great," Holt said. "I think I've got a little bit [of my ancestor's skills] in the bloodline," said Stoker, who spent some time researching the London of 1912 in order to write the book. "We really needed to do the detail the way Bram did - we owed it to him," he said.

"At times we felt in a weird way that Bram was there with us as a third author," added Holt. "We had his notes, and the stories and legends passed down through the family - we were able to give him back his legacy - reclaim Dracula for his roots." Stoker agreed. "Our intent is to give both Bram and Dracula back their dignity. Maybe even more important is to give the novel's legions of loyal fans what they have been waiting over a century for...the return of the real Dracula."

Stoker's original Dracula, the forefather of the wave of vampire novels currently flooding the bookshops, has never been out of print since it was published in 1897. The sequel will be competing with two other high profile vampire novels published next year: film director Guillermo del Toro's debut The Strain, about a vampiric virus which invades New York, and Justin Cronin's The Passage, about a vampire plague spawned by medical experiments.